Trying to build dipole sub - a la Celestion 6000

Siegfried Linkwitz has written extensively on dipoles. Read his web site.

http://www.linkwitzlab.com/

Analog solutions to the dipole roll-off and sub-woofer cross-over are simple and are possible in passive line level form if you have enough gain in your preamp + amplifier.

Note SL's Phoenix prototypes page which has an H-frame with passive dipole equalization and LR2 cross-over to the main speakers.

http://www.linkwitzlab.com/proto.htm

Also note his active filter cookbook

http://www.linkwitzlab.com/filters.htm

Enjoy!

I built Siegfried Linkwitz's Orion design, have run them in three different rooms so far, and have heard a handful of other Orion setups and other dipole bass/sub-bass setups in different places. They work great.

I've also heard Martin Logan's attempts to integrate monopole woofers with dipole main panels and that doesn't work well.

Ngjockey writes
>The reason you don't see much of this is because there's not many crazy enough to try. "Dipole subwoofer" is about as close to an audio oxymoron as an unbiased opinion.

As long as we're being relaxed about defining "sub woofer" like the original poster that's extending the headroom and extension of his electrostatic main panels or the consumer market definition which extends to 80Hz or beyond:

Compared to monopoles there are significant perceptual and measurable differences in how dipole bass couples to room modes (with adjustments possible through source rotation), has no gain below the space's fundamental resonance (why you can't put "big" conventional speakers in small rooms), has a different energy time curve, and has a more accurate modulation transfer function resulting from the speaker/room interface.

People don't do it because it's expensive. In a domestically friendly 14" deep enclosure you need four times the displacement of monopoles for the same SPL at 40Hz and 8X at 20Hz. For music the 40+ Hz output is the issue since even "bass heavy" music has last octave peaks 10-20dB down from the rest of the spectrum with the IEC musical power spectrum approximation specifying a second order high-pass at 40Hz. For mixed home theater use you also need a separate monopole sub-bass system for the lowest frequencies and probably LFE unless you take the Monte Kay approach using 24 15" drivers split into two 9' tall W-frame stacks with a total 1400 pound weight (That does yield 103dB @ 2.83V / 1 meter sensitivity at 18Hz with a 1% second harmonic and 1.5% third harmonic).

That's not very viable commercially in a world where designers lament that $8000 per pair MSRPs only allow for $80 midrange drivers and consumers would need to buy both dipole woofers and monopole sub-woofers for home theater.

>I like the effects I heard with the Celestion but I'm not confident I can reproduce the same results by just mocking up the design. I rather play it safe and go with a tested design.

You can do better. For instance mounting one woofer magnet in and the other magnet out results in a ~15dB reduction in even order harmonic distortion.

If you lack the gear (calibrated measurement microphone, USB interface microphone preamp, software like ARTA) capabilities or motivation (take what you built to an open parking lot and make ground plane measurements) to measure and adjust I'd build a well cooked Linkwitz design, probably one of the H-frame options (Seas drivers per Orion 3.4 with purportedly better large signal performance) since you'll avoid a cavity resonance near your passband. Even with the gear I'd be inclined to share Siegfried Linkwitz's or John Krevosky's driver choices because they've taken the opportunity to listen to and measure a number of drivers' performance in open baffle installations.

>I am concern about the ringing of the woofers as it will be box-less design. It will have to be something that provides servo/braking to keep the bass notes true.

I don't know what your technical concern is here.

People (the Carver Amazing dipole comes to mind) like to use high-Q woofers to compensate for the 6dB/octave dipole roll-off although drivers are minimum-phase devices so where you have flat amplitude you can't have time domain problems.

With a box you get inductance from the air spring in the box in parallel with the suspension compliance which increases both the resonant frequency and Q. Without you just have the driver Thiele-Small parameters and needn't have higher Q.

For sub-woofers the metal cone breakup is so far out of the driver's passband it doesn't matter.

Aerodynamic noises are a bigger issue since you no longer have a box to attenuate them. Pole or cone vents are a fine idea. Voice coil tinsel leads can make noise too but that's at a low enough level that although audible with sine wave test signals it will be masked with music.

04-22-12: Ngjockey
>Personally, the concept of the drivers facing each other seems counter-intuitive for a dipole.

There aren't enough pictures to see what's going on with the Celeston, although opposing drivers are fairly common.

W-frames let you get more displacement out of a given enclosure (spouse approved, or in Monte's case a pair of nine foot towers not four) volume and provide force cancellation to limit structure borne vibrations which could pose problems with floppy floors radiating and sound transmission.

http://www.linkwitzlab.com/woofer.htm

Previously I stated that in a domestically friendly 14" deep enclosure excrusion was 4X that of a monopole and by 20Hz you have 8X the excursion. That's enough to get noticeable movement at high levels in a 70 pound enclosure with a pair of forward facing drivers at the bottom. It's not audible on a rigid floor (radiation from the rocking cabinet is dipolar and therefore rolling off at 6dB/octave due) but could be problematic otherwise.

>The effect would be much the same as a boxed speaker. Ya, I can see how it bring more control to high Q drivers and reduce that boomy sound.

It has nothing to do with a boomy sound.

>The "Direct Servo", along with suitably designed drivers, ameliorates that condition to some degree.

Using electronics you can adjust the system high pass response to pretty much anything you want. Siegfried Linkwitz likes a second order high-pass Q=0.5 with resonance at 20Hz. It sounds good and I've never bottomed my woofers on music (I'm getting 105dB peaks on acoustic recordings with a lot of dynamic range and have had modern music at 95dBC SPL average). John Krevosky has suggested allowing a third order roll-off below system pass-band so subsonics are less an issue (second order acoustic roll-off means excursion is still doubling with each octave lower at a given program material level until you pass the woofer's lowest pole which can be in the single digits for a low-Q woofer). That can be a problem with vinyl though.